Heating circuits of the described kind are known, for instance in xerographic copying apparatus in which the fixing of the toner image that has been transferred from a photoconductor towards a plain paper sheet, occurs by means of a heated pressure-fixing roller. The construction of such circuits requires the provision of a plurality of sliding contacts in order to properly connect the sensor and the electric heater to the electric control circuit that is disposed outside of the roller. Since three, and usually four sliding contacts are required, the construction of the roller becomes rather complicated. Furthermore, the sliding contacts call for regular maintenance. A circuit of the described type is disclosed in U.S. Pat. No. 4,035,612.
Further there is known a controlled heating circuit, wherein a resistive heater element is used to heat a device, the same element being also used as a sensor to determine the actual temperature of the device. This type of heating circuit has the advantage of a simple heating and measuring construction, namely one element that requires only two electrical connections, but it has the disadvantage of a low measuring snesitivity since the sensitivity of a resistance heater is much smaller than the sensitivity of a true sensor, such as a thermistor. Another disadvantage of this heating circuit is that the means which are required for achieving an isolation between the heating circuit and the measuring circuit comprise different components which are rather expensive, and/or frequency-determined. A heating circuit of the described kind is disclosed in U.S. Pat. No. 3,103,573.